As is known, the isotactic polypropylene is endowed with an exceptional combination of excellent properties which render it suitable for a very great number of uses.
In order to improve the properties of the isotactic polypropylene the crystallinity of the propylene homopolymer is decreased by copolymerization of the propylene with small quantities of ethylene and/or α-olefins such as 1-butene, 1-pentene and 1-hexene. In this manner one obtains the so called random crystalline propylene copolymers which, when compared to the homopolymer, are essentially characterized by better flexibility and transparency.
Propylene random copolymers, however, although they have good transparency, do not offer, especially at low temperatures, sufficiently better impact resistance than the homopolymer which can be satisfactory used for the applications listed above.
It has been known for a long time that the impact resistance of polypropylene can be improved by adding an adequate quantity of elastomeric propylene-ethylene copolymer to the homopolymers by mechanical blending or sequential polymerization. However, this improvement is obtained at the expenses of the transparency of the material.
To avoid this inconvenient, U.S. Pat. No. 4,634,740 suggests the blending of the polypropylene, in the molten state, with propylene-ethylene copolymers obtained with specific catalysts, and having an ethylene content ranging from 70 to 85% by weight. However, said compositions present transparency values (Haze) substantially comparable to those of the propylene homopolymer. Said patent, therefore, does not teach how to obtain compositions having good transparency.
EP-A-0557953, describes polyolefin compositions where one obtains a good balance of transparency, stiffness, and impact resistance even at low temperatures, by modifying a crystalline random copolymer of propylene with the proper quantities of a mechanical mixture comprising an elastomeric copolymer and one or more polymers chosen from LLDPE, LDPE and HDPE.
WO 01/92406 describes a propylene polymer composition comprising (percent by weight):                A) from 70 to 90%, of a random copolymer of propylene with ethylene, containing from 1 to 6%, of ethylene, having a content of fraction insoluble in xylene at room temperature of not less than 93;        B) from 10% to 30%, of a copolymer of propylene with ethylene, containing from 8 to 18%, of ethylene;wherein the ratio (B)/C2B of the percent by weight of (B), with respect to the total weight of (A) and (B), to the percent by weight of ethylene in (B), with respect to the total weight of (B), represented in the above formula by C2B, is 2.5 or lower. The MFR L ranges from 0.5 to 50 g/10 min. This composition shows a good transparency but quite low values of flexural modulus.        